Objective To evaluate the angle changes in patients with primary angle closure glaucoma (PACG) after prophylactic laser peripheral iridotomy (LPI) using ultrasound biomicroscopy (UBM), and analyze the treatment effect on patients with different angle closure mechanisms. Methods Basic standard examination and UBM were performed before and 1 week after LPI on 23 patients (35 eyes) who were diagnosed with PACG in The First Affiliated Hospital of Naval Medical University (Second Military Medical University) from Mar. 2018 to Oct. 2021. Based on the UBM results, the angle closure mechanisms were divided into pupillary block type, plateau iris type, and mixed type. Results Among the 35 eyes, 30 (85.7%) eyes showed pupillary block type and 5 (14.3%) showed plateau iris type before LPI. After LPI, 18 of the 30 eyes showing pupillary block before LPI showed pupillary block type and 12 showed mixed type, and the degree of angle opening was increased (20.2°±4.2° vs 6.6°±1.8° in reclassified pupillary block type and 12.4°±3.9° vs 6.3°±1.7° in reclassified mixed type, both P<0.05); the 5 eyes showing plateau iris type before LPI had no change in classification or the degree of angle opening (8.3°±1.7° vs 6.7°±2.1°, P>0.05). Conclusion The effect of opening angle after LPI is better in PACG patients with pupillary block type, but not in patients with plateau iris type, suggesting that plateau iris plays an important role in angle closure mechanisms in PACG patients and affects the effect of LPI.